It'd be possible to dispense with the per-buffer spinlocks so long as
you look only at the tag (and perhaps the TAG_VALID flag bit). The
tags can't be changing while you hold the BufMappingLock.
That's what I had thought at first, but this comment in buf_internals.h dissuaded me: "buf_hdr_lock must be held to examine or change the tag, flags, usage_count, refcount, or wait_backend_id fields." The comment already notes this isn't true if you've got the buffer pinned; it would be worth adding another exception for holding the BufMappingLock, IMHO.
I'm dubious that there's any point in recording information as transient as the refcounts and dirtybits
I think it's worth recording dirty bits -- it provides an indication of the effectiveness of the bgwriter, for example. Reference counts could be done away with, although I doubt it would have a significant effect on the time spent holding the lock.
-Neil
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